The Viral Analysis Unit of the LMMN has evaluated over 3500 clinical samples including CSF, plasma and serum for the detection of JC Virus genome sequences. This finding is diagnostic for PML, a rare but mostly fatal disease of the brain, as well as indicating the viral course of infection in many tissue compartments. The LMMN receives many such samples from around the world in circumstances in which diagnosis and pathogenesis cannot be made on clear clinical findings alone. The VAU has made hundreds of determinations on such samples and made the diagnosis on complex cases that provided critical information to neurologists and their patients for future medical care and treatment. In a survey of hundreds of plasma samples, it became clear that over 2% of the population worldwide can be viremic and that approximately 1% of the population is persistently viremic. Such numbers in a well defined and reliable assay system. The nature of the treatment that such viremic patients may receive for underlying diseases particularly that are classified as autoimmune diseases i.e. Multiple Sclerosis, Rheumatoid arthritis, Crohn's disease, Systemic Lupus Erythymatosis. Of the 165 MS patients that developed PML while treated with natalizumab,over half were diagnosed with the data from the Viral Analysis Unit. There have been at least 16 of these patients that had been misdiagnosed and not given correct medical treatment until the VAU provided the correct evidence of PML. Also the cumulative longitudinal data from this work has established that many PML patients never clear their infection even if immune reconstitution is establihsed. In some cases, patients show clinical signs of disease years after the initial diagnosis. This occurs in both HIV/AIDS cases as well as MS cases of PML and warrants close neurological evaluation at annual or sooner time points.